Do I qualify for a visa to Thailand as a sole employee of my own company in Bulgaria, and what financial documents do I need?

I would say the required documents should be your recent utility bill for current location and for freelancer status company incorporation docs along with a signed and stamped letter head stating that you can work from thailand, your personal bank accounts containing equivalent to 500k bt, also you can supplement with your company's statement, previous years tax return, invoices/ledger from from trading account should be sufficient, I know its a over kill but better be safe than sorry. If you are doubtful then you can go for soft power route, just search the group. No one can guarantee visa as different embassies have different policies. Also some embassies demand 500k bt equivalent is maintained through /1 month and few are okay with only last balance in your personal bank account.

get 500k tbh in personnal account and it should be fine, note some embassady want to check that you have that money in your personnal account since 3 or 6 months ...

Look at the Thai Embassy in Bulgaria website.

Best to apply at the Embassy in the country where you have residency.

Sometimes it depends purely on the consulate you intend to apply at. Overkill is better than missing the mark.